While several treatment modalities have proven effective for cancer immunotherapy, cancer immunotherapists will undoubtedly need more than one weapon in their therapeutic armamentarium. In particular, different approaches are required for tumors with high vs. low mutation loads.1 Solid tumors induced by carcinogens (e.g., melanoma, lung cancer) express numerous mutations that create tumor-specific antigens (TSAs) which can be targeted using two approaches: injection of ex vivo expanded tumor-infiltrating lymphocytes and administration of antibodies against checkpoint molecules.1-3 However, TSAs are exceedingly rare on hematologic cancers (HCs), because of their very low mutation load, and alternative targets must therefore be found for immunotherapy of HCs.1 T cells redirected to CD19 or CD20 antigen targets with engineered chimeric antigen receptors are spectacularly effective for treatment of B-cell malignancies and represent a breakthrough in cancer immunotherapy.4,5 However, whether chimeric antigen receptors might be used for treatment of myeloid malignancies remains a matter of speculation.6 
Major histocompatibility complex (MHC) molecules are transmembrane glycoproteins encoded by closely linked polymorphic loci located on chromosome 6 in humans. Their primary role is to bind peptides and present them to T cells. MHC molecules (HLA in humans) present thousands of peptides at the surface of human cells. These MHC-associated peptides (MAPs) are referred to as the immunopeptidome. The immunopeptidome of identical twins (AKA syngeneic individuals) is identical. By contrast, MAPs present on cells from HLA-identical non-syngeneic individuals are classified into two categories: i) monomorphic MAPs which originate from invariant genomic regions and are therefore present in all individuals with a given HLA type, and ii) polymorphic MAPs (AKA MiHAs) which are encoded by polymorphic genomic regions and are therefore present in some individuals but absent in other individuals. MiHAs are essentially genetic polymorphisms viewed from a T-cell perspective. MiHAs are typically encoded by bi-allelic loci and where each allele can be dominant (generates a MAP) or recessive (generates no MAP). Indeed, a non-synonymous single nucleotide polymorphism (ns-SNP) in a MAP-coding genomic sequence will either hinder MAP generation (recessive allele) or generate a variant MAP (dominant allele).
Another strategy that can be used for cancer immunotherapy is adoptive T-cell immunotherapy (ATCI). The term “ATCI” refers to transfusing a patient with T lymphocytes obtained from: the patient (autologous transfusion), a genetically-identical twin donor (syngeneic transfusion), or a non-identical HLA-compatible donor (allogeneic transfusion). To date, ATCI has yielded much higher cancer remission and cure rates than vaccines, and the most widely used form of cancer ATCI is allogeneic hematopoietic cell transplantation (AHCT).
The so-called graft-versus-leukemia (GVL) effect induced by allogeneic hematopoietic cell transplantation (AHCT) is due mainly to T-cell responses against host MiHAs: the GVL is abrogated or significantly reduced if the donor is an identical twin (no MiHA differences with the recipient) or if the graft is depleted of T lymphocytes. More than 400,000 individuals treated for hematological cancers owe their life to the MiHA-dependent GVL effect which represents the most striking evidence of the ability of the human immune system to eradicate neoplasia. Though the allogeneic GVT effect is being used essentially to treat patients with hematologic malignancies, preliminary evidence suggests that it may be also effective for the treatment of solid tumors. The considerable potential of MiHA-targeted cancer immunotherapy has not been properly exploited in medicine. In current medical practice, MiHA-based immunotherapy is limited to “conventional” AHCT, that is, injection of hematopoietic cells from an allogeneic HLA-matched donor. Such unselective injection of allogeneic lymphocytes is a very rudimentary form of MiHA-targeted therapy. First, it lacks specificity and is therefore highly toxic: unselected allogeneic T cells react against a multitude of host MiHAs and thereby induce graft-versus-host-disease (GVHD) in 60% of recipients. GVHD is always incapacitating and frequently lethal. Second, conventional AHCT induces only an attenuated form of GVT reaction because donor T cells are not being primed (pre-activated) against specific MiHAs expressed on cancer cells prior to injection into the patient. While primed T cells are resistant to tolerance induction, naïve T cells can be tolerized by tumor cells.
It has been demonstrated in mice models of AHCT that, by replacing unselected donor lymphocytes with CD8+ T cells primed against a single MiHA, it was possible to cure leukemia and melanoma without causing GVHD or any other untoward effect. Success depends on two key elements: selection of an immunogenic MiHA expressed on neoplastic cells, and priming of donor CD8+ T cells against the target MiHA prior to AHCT. A recent report discusses why MiHA-targeted ATCI is so effective and how translation of this approach in the clinic could have a tremendous impact on cancer immunotherapy8.
High-avidity T cell responses capable of eradicating tumors can be generated in an allogeneic setting. In hematological malignancies, allogeneic HLA-matched hematopoietic stem cell transplantation (ASCT) provides a platform for allogeneic immunotherapy due to the induction of T cell-mediated graft-versus-tumor (GVT) immune responses. Immunotherapy in an allogeneic setting enables induction of effective T cell responses due to the fact that T cells of donor origin are not selected for low reactivity against self-antigens of the recipient. Therefore, high-affinity T cells against tumor- or recipient-specific antigens can be found in the T cell inoculum administered to the patient during or after ASCT. The main targets of the tumor-reactive T cell responses are polymorphic proteins for which donor and recipient are disparate, namely MiHAs.
However, implementation of MiHA-targeted immunotherapy in humans has been limited mainly by the paucity of molecularly defined human MiHAs. Based on the MiHAs currently known, only 33% of patients with leukemia would be eligible for MiHA-based ATCI. MiHA discovery is a difficult task because it cannot be achieved using standard genomic and proteomic methods. Indeed, i) less than 1% of SNPs generate a MiHA and ii) current mass spectrometry methods cannot detect MiHAs.
Thus, there is a need for the identification of novel MiHAs that may be used in immunotherapies.
The present description refers to a number of documents, the content of which is herein incorporated by reference in their entirety.